Introduction

0

Recently there has been increased interest in the use of high-flow nasal cannula (HFNC) to provide preoxygenation and apneic oxygenation during endotracheal intubation.Previous posts have discussed the basic physiology and some evidence behind this.Vourc'h et al. just published a RCT showing no benefit from HFNC in this situation (1).What should we make of this new data?

0

Overview of Vourc'h et al.

0

0

The study design is shown above. 124 hypoxemic patients requiring intubation were randomized to either a HFNC group (who received 60 liter/min flow of 100% oxygen throughout the procedure) or a control group (who received preoxygenation using a face mask at 15 liters/minute flow and no apneic oxygenation).The primary endpoint was the lowest saturation during the intubation procedure.There was no significant difference in this outcome, with a trend towards improved saturation in the HFNC group (figure below).This was a very sick group of patients, who experienced a substantial rate of severe desaturation.

0

0

Consideration of these results in context of physiology

0

These results make little physiologic sense.The control group was preoxygenated with a facemask at 15 liters/minute flow (which typically achieves an inhaled FiO2 of 60-70%; Weingart and Levitan 2011) and received no apneic oxygenation.In contrast, the HFNC group was preoxygenated with 100% oxygen at 60 liters/minute flow (which provides >90% inhaled FiO2 as reviewed here) and received ample amounts of apneic oxygenation.

0

In order to believe the results of this study, one would have to question both the utility of HFNC and also the utility of apneic oxygenation.Based on physiology and prior evidence supporting apneic oxygenation, there really should have been no contest between these two therapeutic arms.

0

Why didn't they observe a difference?

0

Desaturation during endotracheal intubation depends on a number of factors, including the quality of preoxygenation and apneic oxygenation, procedure duration, severity of underlying lung disease, and lung collapse during the procedure.For example, an edentulous patient who can be intubated in 15 seconds may not desaturate despite poor preoxygenation.Alternatively, a patient with severe ARDS and morbid obesity may desaturate despite ideal preoxygenation and apneic oxygenation.

0

One challenge of critical care studies is that patients are very heterogeneous.Although higher quality preoxygenation may make a difference, this difference will be very hard to detect in the setting of widely varying patients with differing illness severity and airway anatomy.It is likely that any signal from different types of preoxygenation was lost in this “noise” induced by heterogeneity.The authors of this study did concede that “the timing of invasive mechanical ventilation probably governs the depth of desaturation during [intubation] more than the preoxygenation device.”

0

0

Conclusions

0

This study failed to detect a benefit from HFNC as well as apneic oxygenation, most likely due to a relatively low sample size combined with a high degree of patient heterogeneity.It is extremely difficult to believe that neitherHFNC nor apneic oxygenation work at all.

0

This study does emphasize an important point, which is that providing 100% FiO2 cannot prevent desaturation due to atelectasis and derecruitment of the lungs (which causes a physiologic shunting of blood through collapsed lung areas).Patients at higher risk of lung collapse include patients with morbid obesity and patients with parenchymal lung disease (e.g. ARDS).Such patients may be most safely preoxygenated using noninvasive ventilation in order to provide both high levels of FiO2 and positive pressure to recruit the lungs.

0

0

Vourc'h et al. found that neither HFNC nor apneic oxygenation were effective for reducing desaturation during intubation.This is probably due to a high level of heterogeneity among patients, drowning any potential signal in noise.

This study should notbe used as evidence to abandon HFNC and apneic oxygenation to reduce peri-intubation desaturation.In particular there is an extensive body of evidence supporting the efficacy of apneic oxygenation (e.g., Weingart and Levitan 2011). The precise role of HFNC remains unclear.

Providing 100% FiO2 cannot prevent desaturation due to lung collapse. For patients at high risk of lung collapse (e.g. ARDS, morbid obesity), noninvasive ventilation should be considered since it provides both positive pressure and high FiO2.

Share this:

2

Comment Here

Role

2Comment threads

0Thread replies

0Followers

Most reacted comment

Hottest comment thread

2Comment authors

Recent comment authors

Role

newestoldestmost voted

Guest

fraz65

Apnoeic oxygenation makes physiological sense, however the Weingart/ Levitan paper only cites two references. Both of these studies enrolled only fifteen patients in each arm. I still apply nasal oxygen during RSI but I’m still to be convinced by the evidence base for it.

When I read these studies claiming that ApOx is no better than standard care it gives my brain a cramp because I’ve seen the benefits over the course of my career as a paramedic. When I really try to think through the physiology and process of preoxygenation and apnic oxygenation, I’m left with a few comments and questions. First, it makes sense that a patient with severe pulmonary disease might not benefit from HFNC over HFFM because of thier shunt physiology, unless the shunt is atleast partially corrected. Presumably, neither HFFM nor HFNC can acheive enough alveolar pressure to “pop open” collapsed alveoli. Thats where NIPPV comes in, however you decide to deliver this. Personally I’ve seen the BVM + PEEP valve + HFNC combo work very nicely in the prehospital arena, however the use of a ventilator I’m sure would be even more effective. So, that brings me to my first question. What is the actual alveolar pressure necessary to open collapsed alveoli, both in the healthy patient and in the patient with lung pathology such as ARDS or emphysema? We all know that the pressure necessary to maintain open alveoli is far less than the pressure needed to… Read more »